The present invention relates to oxygen scavenging compositions, to polymeric compositions containing said oxygen scavenging compositions, and further to intermediate shaped structures, e.g., films, coatings, 3-dimensional solids, fibers, webs, and the like, which contain such polymeric compositions, as well as to shaped products, into or onto, which such compositions or structures are incorporated or applied, respectively, e.g., packaging articles, having the subject compositions incorporated as part of or attached to the article's structure.
It is well known that limiting the exposure of oxygen-sensitive materials to oxygen maintains and enhances the quality and "shelf-life" of the material. For instance, by limiting the oxygen exposure of oxygen sensitive food products in a packaging system incorporating a material or composition capable of scavenging oxygen, the quality of the food product is maintained and food spoilage is avoided for extended periods. In addition, such packaging systems permit keeping the product in inventory longer and, thereby, reduce costs incurred from waste and having to restock. In the food packaging industry, several means for limiting oxygen exposure have already been developed. At present, the most commonly used means are modified atmosphere packaging (MAP), and vacuum packaging coupled with the use of oxygen barrier films. In these instances, reduced oxygen environments are employed at the time of packaging and the oxygen barrier film physically prevents oxygen from entering the packaging environment during storage.
Another, more recent means for limiting oxygen exposure involves incorporating an oxygen scavenger into the packaging structure. The term "oxygen scavenger" or "scavenger", as used in the present specification and appended claims refers to compounds and compositions which are capable of consuming, depleting or reducing the amount of oxygen from a given environment. Incorporation of a scavenger in a package (e.g., as part of a film forming the package, or at least one layer of a laminate forming the package or as a coating on at least a portion of the package structure) can provide a means of scavenging oxygen in the headspace of the package as well as providing uniform scavenging effect throughout the package. In addition, incorporation of a scavenger can provide a means of intercepting and scavenging oxygen as it is passing through the walls of the package (herein referred to as an "active oxygen barrier") to maintain the lowest possible oxygen level throughout the package.
Examples of oxygen scavenger compositions incorporated into an oxygen scavenging wall are illustrated in European Applications 301,719 and 380,319; PCT 90/00578 and 90/00504, and U.S. Pat. Nos. 5,021,515 and 5,049,624. The oxygen scavenger compositions disclosed in these publications comprises a polyamide and a transition metal catalyst. A package wall containing such compositions regulate the amount of oxygen which reaches the interior of the package. However, the onset of useful oxygen scavenging activity, i.e. up to about 5 cubic centimeters (cc) oxygen per square meter per day at ambient conditions, by this wall may not occur for as long as 30 days. The delay before the onset of useful oxygen scavenging is hereinafter referred to as the induction period. Such extended induction period is not generally desired.
Other oxygen scavenger compositions comprising a transition metal catalyst and an ethylenically unsaturated compound, e.g. polybutadiene, polyisoprene, dehydrated castor oil, etc., as described in U.S. Pat. No. 5,346,644, also exhibit lengthy induction periods. For example, when the oxygen scavenger comprises a polybutadiene, the induction period can exceed thirty days. Scavengers comprising polyisoprene or dehydrated castor oil typically have induction periods of about one to fourteen days. The duration of the induction period depends on several factors, some of which are not completely understood or controllable. Accordingly, when using films or articles containing oxygen scavenger compositions having long induction periods, it is required to keep the films and articles in inventory for a period of time prior to use in order to provide reliable scavenging behavior required to protect oxygen sensitive material in a package. On the other hand, when using packages which incorporate films or articles containing scavenger compositions having short induction periods, the package, films and articles, as appropriate, will have to be prepared quickly and put to use in a short time period, sometimes immediately or stored in an oxygen-free atmosphere in order to attain the maximum effectiveness as a scavenger.
One method described to initiate scavenging on demand in packages for oxygen-sensitive foods or other materials involves incorporating photooxidizable rubber, e.g. cis-1,4-polyisoprene, and a photosensitizing dye into the inner surface of a package and then exposing it to visible light. See Rooney, M. L., "Oxygen Scavenging: A Novel Use of Rubber Photo-oxidation", Chemistry and Industry, Mar. 20, 1982, pp. 197-198. However, while this method allows one to initiate oxygen scavenging when desired, it requires constant exposure of the package to light to maintain the scavenging effect. Such a requirement is not suitable for commercial application. Further, the required use of a dye makes it difficult to employ this method for applications which require colorless packaging, especially the transparent packaging commonly used commercially with food and beverage products.
A method of initiating oxygen scavenging by compositions which comprise oxidizable organic compounds and transition metal catalysts is disclosed in U.S. Pat. No. 5,211,875, which is incorporated herein by reference as if set forth in full. The reference sets forth a method of initiating oxygen scavenging by administration of a dose of actinic radiation. The oxygen scavenging compositions are conveniently prepared by compounding a scavenging resin(s), transition metal catalyst and, optionally, a photoinitiator. The scavenging is initiated by subjecting the package, film or article containing the scavenging compositions to actinic radiation. However, the oxygen scavenging compositions prepared by this reference exhibit an undesirably high level of migration of the photoinitiator and/or its by-products from the packaging material, particularly when used to package fatty substances.
It is highly desired to provide an improved oxygen scavenging composition suitable for use in packages, films and articles. The composition should provide the ability to have useful oxygen scavenging activity within short induction periods after irradiation. Further, the composition should be capable of retaining the active components and the irradiation by-products within a polymer matrix used as a carrier for the scavenging composition.